2 (d) Rotation

Earth moves through space in two major ways: rotation and revolution. The spinning of Earth on its axis is called rotation. It takes Earth about 24 hours to rotate once. Earth also moves around the sun. In addition to rotating on it axis Earth travels around the sun. Revolution is the movement of one object around another. One complete revolution of Earth around the sun is called a year. Earth follows a path, or orbit, as it revolves around the sun. Earth’s orbit is not quite circular. It is a slightly elongated circle, or ellipse.Earth orbits the sun in about 365 ¼ days. If the sun and Earth are constantly pulling on one another because of gravity, why doesn’t Earth fall into the sun? Similarly, why doesn’t the moon crash into Earth? The fact that such collisions have not occurred shows that there must be another factor at work. That factor is called inertia. The tendency of an object to resist a change in motion is inertia. You feel the effects of inertia every day. When you are riding in a car and it stops suddenly, you keep moving forward. If you didn’t have a seat belt on, your inertia could cause you to bump into the car’s windshield or the seat in front of you. Why do Earth and the moon remain in their orbits? Newton concluded that two factors—inertia and gravity—combine to keep Earth in orbit around the sun and the moon in orbit around Earth. Earth revolves around the sun because the sun’s gravity pulls on it while Earth’s inertia keeps it moving ahead. The imaginary line that passes through Earth’s center and the North and South poles is Earth’s axis. The spinning of Earth on its axis is called rotation. Earth’s rotation causes day and night. As Earth rotates eastward, the sun appears to move westward across the sky. But is the sky really moving above you? Earth’s rotation causes day and night. As Earth rotates eastward, the sun appears to move westward across the sky. It is day on the side of Earth facing the sun. As Earth continues to turn to the east, the sun appears to set in the west. Sunlight can’t reach the side of Earth facing away from the sun, so it is night there. It takes Earth about 24 hours to rotate once. As you know, each 24-hour cycle of day and night is called a day.Most places outside the tropics and polar regions have four distinct seasons: winter, spring, summer, and autumn. But there are great differences in temperature from place to place. For instance, it is generally warmer near the equator than near the poles. Notice that sunlight hits Earth’s surface most directly near the equator. Near the poles, sunlight arrives at a steep angle. As a result, it is spread out over a greater area. That is why it is warmer near the equator than near the poles.If Earth’s axis were straight up and down relative to its orbit, temperatures would remain fairly constant year-round. There would be no seasons.

The equator has a latitude of 0 degrees and it separates the Northern and Southern hemisphere. In general, light from the sun is less direct as you increase in latitude. To increase in latitude is to move away from the equator and toward the North pole. During our summer the Northern hemisphere is tilted toward the sun. Our days are longer and our shadows are shorter, because the sun gets higher in the sky and is more over our heads.

If Earth’s axis were straight up and down relative to its orbit- there would be no
seasons. Earth has seasons because its axis is tilted as it revolves around the sun. Earth’s axis is always tilted at an angle of 23.5° from the vertical. As Earth revolves around the sun, the north end of its axis is tilted away from the sun for part of the year and toward the sun for part of the year. Summer and winter are caused by Earth’s tilt as it revolves around the sun. The change in seasons is not caused by changes in Earth’s distance from the sun. In fact, Earth is farthest from the sun when it is summer in the Northern Hemisphere.